It is known that different frequencies of light do not focus at the same location after passing through a lens. For this reason, color correction has been built into prior art lens systems. This is true not only for lens systems that focus visible light, but also for infrared lens systems. A typical infrared (IR) system operates over a moderate waveband, such as approximately.times.3-5 microns or 8-12 microns. Additionally, most lenses, particularly lenses with spherical surfaces, introduce field aberrations including astigmatism and coma.
FIG. 1 shows a conventional single field-of-view (FOV) imager. It includes 3 refractive elements, the objective 10, color corrector lens 12, and a field lens 14. The objective lens 10 is the primary focusing element which has the most power and is a converging lens. The objective lens collects the light from the desired object to be imaged and focuses this energy onto the detector. The objective lens is commonly made from germanium, due to germanium's high refractive index and low dispersion. Dispersion is the variation of refractive index with wavelength causing each wavelength to focus at a slightly different location. For an 8-12 micron waveband, these multiple foci would cause a blurring of the image. Thus, a color correcting lens is needed. The color corrector lens 12 is a negatively powered highly dispersive material used to bring all desired wavelengths of light to a common focus. The color corrector lens is typically made from zinc selenide. The field lens 14 is a positively powered lens used to correct field or image aberrations such as astigmatism and/or coma. The field lens is typically made from germanium.